The historical consensus holds that the invention of the stirrup was a major development in military history. By permitting the horseman to keep his seat, the simplified story goes, the stirrup changed the dominant strategy from the infantry-based armies of antiquity to the shock cavalry-based armies that came to dominate in the middle ages.
This story seems to make sense. The change in the composition of European armies is real and needs to be explained. (Infantry remained the numerical majority of most armies, but heavy cavalry became more important in determining the outcome of battles.) Horsemen without stirrups used different equipment in different ways than the stirrup-using knights we’re familiar with. The cavalry charge against massed infantry is almost unheard of in antiquity but becomes an extremely important tactic from the early middle ages until well after the ubiquity of firearms.
However, there is no historical consensus on when the stirrup became important in Europe. I’ve seen serious claims ranging from the late 300s to the late 700s. There’s sharp disagreement over very basic claims, like “Was the Battle of Adrianople a triumph of cavalry over infantry?” or “Did the Carolingian military use stirrups?” (I haven’t checked whether these questions were resolved by recent archeological work, but if the answers weren’t obvious 40 years ago, that’s still a notable fact.) The history of the stirrup before it reached Europe, e.g. in India or central Asia, is no clearer.
This is super weird. If the stirrup was such a huge deal, shouldn’t we be able to see its effects? If a historian in the year 3000 were trying to date the advent of the machine gun, and only had fragments of secondary sources and doubtful archeological scraps, it would still be possible because the machine gun so greatly transformed strategy, tactics, and the experience of individual soldiers. (The American Civil War is the only case I can think of where a smart scholar might get the wrong answer.) This is what we see for other massive shifts in historical weapons, such as chariots, castles, and artillery. If the stirrup were anywhere near this important, its effects should be similarly visible.
I’ve read all these historians arguing about the minutiae of manuscripts and archeological finds to set dates on when the stirrup was used where, but if their basic claim about the importance of the stirrup is true, then there should be much simpler avenues to answering the question.
At this point, I’m inclined to think the stirrup was not as overpowering as is commonly asserted. Important, yes, but important on the scale of chainmail or the rifled barrel, not on the level of the phalanx or the nuclear bomb. Not important enough to explain the transition from armies dominated by infantry to armies dominated by cavalry. If it were, its history would be more apparent.
If true, this raises two questions. The first, why so many historians have overstated its importance, is relatively easy to answer. For one thing, contemporary prejudices favor explaining large-scale trends as the natural consequence of technological development. More importantly, historians are like anyone else in that they are biased towards simple and compelling explanations for things. The story of the stirrup transforming combat has enough truth to it to lay the foundation for such a narrative. It fits very well from a purely local perspective. In contrast, broad sociological outside-view checks like the one I’m running here seem, if not rare, then at least uncommon.
The more difficult question is why Europe transitioned from infantry-based armies to cavalry-based armies, if not for stirrups. I’m not sure. It could be a combination of technological factors: larger horses, improved saddles, better armorsmithing, horseshoes, and the temporary loss of the composite bow, together with stirrups, producing a combined effect greater than the sum of its parts. It’s possible, but I don’t trust this type of explanation. Strategic considerations are usually Pareto-distributed in importance, and one major factor tends to overwhelm many medium-size factors.
It could be a matter of economic and social organization: the sharp division between landholding knights extracting wealth from their tenants on the one hand, and peasant farmers with little capital on the other, led to a combination of arms that was perhaps inefficient from a purely military standpoint but crucial with regard to internal coherence, thus leading Carolingian-style feudalism to succeed and spread in spite of some necessary overemphasis on heavy cavalry. This strikes me as plausible but far from certain. The institutional and cultural prominence of knighthood in Europe is consistent with this story, at the very least.
A more exotic version of the prior hypothesis is that Europe transitioned to cavalry not because its cavalry was strong, but because its infantry was weak. If feudalism made it institutionally and ideologically difficult to raise large masses of competent, well-equipped infantry, then perhaps this explains the shift.
I’m not confident in any of these explanations. The more I look into this, the more I think the dominance of cavalry in medieval Europe is a mystery that still needs to be explained.
Edit: Stephen Morillo can explain the dominance of cavalry. See his comment and full article. Turns out the “Infantry was weak” hypothesis is correct.